1. Field of the Invention
The invention relates to a head-mounted display provided with an image display device and an optical system comprising a concave mirror and a semi-transmissive element for projecting an image to be formed by the image display device on a user""s retina via, consecutively, the semi-transmissive element, the concave mirror, the semi-transmissive element and an exit pupil of the head-mounted display.
2. Description of the Related Art
A device of the type described above is known from the English-language abstract of Japanese patent application 09090312 A. In a head-mounted display described in this document, an image is formed by means of a liquid crystalline image display panel, which image is projected in the exit pupil of the head-mounted display by means of an optical system comprising a splitting mirror and a concave mirror. During use, the exit pupil coincides with the eye pupil of a user of the head-mounted display, so that an image is formed on the retina. The head-mounted display may be worn by a user for displaying, for example, TV or video images or pictures associated with portable information and communication systems, computer games or computer simulations.
The known head-mounted display has a comparatively low cut-off frequency of the optical modulation transfer function so that the use of an image display device with a high resolution of, for example, a frame matrix of 1024xc3x97768 pixels with a diagonal dimension of 0.5 inch is hardly useful.
It is an object of the invention to improve the optical modulation transfer function. To this end, the device according to the invention is characterized in that the head-mounted display comprises a first lens which is arranged between the image display device and the semi-transmissive element for collimating radiation to be generated by the image display device in the optical system. Since the first lens collimates the radiation to be generated, coming from the image display panel, the radiation is concentrated along an optical axis of the optical system so that the distortion is reduced and the cut-off frequency of the optical modulation transfer function is increased. A further advantage of placing the lens is that the distortion in the corners of the image is reduced.
A particular embodiment of the head-mounted display according to the invention is characterized in that the first lens has a convex surface and a substantially plane surface, and the convex surface faces the semi-transmissive element. It appears that, with such an arrangement, the optical modulation transfer function is further improved.
A further embodiment of the head-mounted display according to the invention is characterized in that the optical system comprises a xcex/4 plate which is arranged between the semi-transmissive element and the concave mirror, and the semi-transmissive element comprises a polarization-dependent beam splitter for reflecting radiation polarized in a first direction and to be generated by the image display device, and for selectively passing radiation polarized in a second direction and coming from the concave mirror and the xcex/4 plate. The use of the xcex/4 plate and the polarization-dependent beam splitter reduces the background illumination and enhances the transmission of the radiation generated by the image display device.
A further embodiment of the head-mounted display according to the invention is characterized in that the polarization-dependent beam splitter comprises two prisms which are separated by a beam-splitting surface having a polarization beam-splitting layer for reflecting the radiation polarized in the first direction and to be generated by the image display device, and for selectively passing the radiation polarized in the second direction and coming from the concave mirror and the xcex/4 plate. Such a polarization beam-splitting layer is known from, for example, the handbook xe2x80x9cThin Film Optical Filtersxe2x80x9d as published by H.A. Macleod, p. 328, section 33. This polarization beam-splitting layer comprises, for example, a stack of optical layers which alternately have a high and a low refractive index. By using such a polarization-dependent beam splitter, a transmission of substantially 100% is achieved for the radiation coming from the image display device. A further advantage is that a thickness of the head-mounted display is reduced, which thickness is determined by the distance between the concave mirror and the exit pupil. The use of a xcex/4 plate and a polarization-dependent beam splitter in a head-mounted display is known per se from United States patent U.S. Pat. No. 5,771,124.
A further embodiment of the head-mounted display according to the invention is characterized in that the polarization-dependent beam splitter comprises a semi-transmissive reflector and a polarization-dependent reflector for reflecting radiation polarized in the first direction and to be generated by the image display device, and for selectively passing the radiation polarized in the second direction. The use of the polarization-dependent beam splitter of this second type is advantageous because it provides a construction of the head-mounted display with a lower weight as compared with the weight of the above-mentioned head-mounted display of a first type which comprises the prisms. It is to be noted that the transmission of the radiation of the head-mounted display of the second type, coming from the image display device, is a factor of eight lower than a corresponding transmission of the head-mounted display of the first type.
A further embodiment of the head-mounted display according to the invention is characterized in that the semi-transmissive reflector has an angle-dependent reflective layer which has a relatively high reflection for large angles of incidence and a relatively low reflection for small angles of incidence. Such a layer comprises a packet of sub-layers which alternately comprise a sub-layer of optically dense material and a sub-layer of optically less dense material, the thickness of the sub-layer being equal to half the wavelength of the radiation applied.
The transmission of the head-mounted display is enhanced by using such a layer.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises a second lens which is arranged between the semi-transmissive element and the exit pupil for prefocusing the radiation from the semi-transmissive element to the exit pupil of the head-mounted display.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises at least an optical element having an aspherical surface. The use of optical elements with aspherical surfaces reduces imaging errors such as coma, astigmatism and curvature of the field.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.